Improved process of coating ferrous metal strands

ABSTRACT

METALLIC COATING METHOD AND APPARATUS WHEREIN A STRAND IS WITHDRAWN FROM A BATH OF MOLTEN COATING METAL PAST A ROLL ONLY PARTLY SUBMERGED IN THE BATH AND ROTATING SO AS TO PROVIDE AN OXIDE FREE BATH SURFACE THROUGH WHICH THE STRAND EMERGES FROM THE MOLTEN METAL. THIS ROLL MAY BE POSITIONED SO AS TO EFFECT A BENDING OF THE STRAND AS IT EMERGES FROM THE BATH.   D R A W I N G

Jan. 16, 1973 F. BYRD ETAL 3,711,320

IMPROVED PROCESS OF COATING FERROUS METAL STRANDS Filed Jan. 8, 1971 INVENTORS FRED BYRD PAUL E SCHNEDLER BY kyzad Jrz a/za Zia/1 ATTORNEYS United States Patent 3,711,320 IMPROVED PROCESS OF COATING FERROUS METAL STRANDS Fred Byrd, Lebanon, and Paul E. Schnedler, Middletown,

Ohio, assignors to Armco Steel Corporation, Middletown, Ohio Filed Jan. 8, 1971, Ser. No. 104,938 Int. Cl. C23c 1/00 US. Cl. 117102 M 4 Claims ABSTRACT OF THE DISCLOSURE Metallic coating method and apparatus wherein a strand is withdrawn from a bath of molten coating metal past a roll only partly submerged in the bath and rotating so as to provide an oxide free bath surface through which the strand emerges from the molten metal. This roll may be positioned so as to effect a bending of the strand as it emerges from the bath.

BACKGROUND OF THE INVENTION This invention relates to a method and apparatus for the application of a molten metallic coating to a ferrous base metal strand. The invention has great and particular utility in connection with a metallic coating operation wherein the finishing of the molten metal adhering to the strand is accomplished by an elongate gaseous jet directed at the molten coating metal adhering to the strand.

All of the metallic coating operations of the type under consideration contemplate the maintenance of a pot or bath containing the molten coating metal. By way of example, the commonly used coating metals include terne, zinc, aluminum or alloys thereof. When maintained in the molten state in a pot or coating metal bath, all of these metals form a relatively heavy oxide scum or dross at the surface. It follows that when the strand being coated is withdrawn upwardly from the coating metal bath, it must pass through this oxide scum.

Various techniques have been utilized by prior art to prevent this scum or dross from being drawn upwardly by the strand as it emerges from the bath. For example, it is known in the art to utilize a pair of exit rolls partly submerged in the bath of molten coating metal and arranged to contact each side of the strip as it emerges from the bath. According to conventional techniques, the weight of coating metal applied to the strand may be controlled by varying slightly the position of the rolls with respect to the surface of the coating bath 4 or the pattern on the rolls. Uniformity or contour of the coating is controlled by the alignment and/or contour of the exit rolls. Since the strand being coated was emerging from the bath at a point between the two rolls, the problem of oxide or dross being carried upwardly by the strand was minimal. However, it is now recognized by the skilled worker in the art that metallic coating operations utilizing a mechanical finishing technique such as exit rolls must be operated at a relatively low speed.

It is now generally believed that a metallic coating operation can be carried out at much higher speeds by use of an elongate gaseous jet to finish the molten coating metal adhering to the strand. According to this practice, the strand emerges from the bath, and through the oxide or scum layer referred to earlier. It is well recognized that this problem is particularly acute at the edges of the strand.

In addition, the weight and contour of the metallic coating must be controlled by the action of the gaseous jet. In order to achieve satisfactory results, it is necessary that the strand being coated be as flat as possible when 3,711,320 Patented Jan. 16, 1973 it passes between the opposed gaseous jets. To achieve uniform coating weights on opposite sides of the strand, it must be centered between opposed gaseous jets.

In the light of the above comments, it is an object of this invention to provide a method and apparatus which is effective to prevent oxide and dross from being drawn upwardly by the strand as it emerges from the coating metal bath.

Another object of this invention is to provide a method and apparatus which will minimize the normally heavy coating metal concentration and oxides found at the edges of a'strip.

Still a further object of this invention is to present a very flat strip to the gaseous finishing nozzles, thereby improving coating weight distribution.

SUMMARY OF THE INVENTION Broadly considered, this invention is based upon the surprising discovery that oxide and berries on the strip edges are eliminated by positioning a roll partly submerged in the bath and rotating the roll so as to provide an oxide free bath surface through which the strand emerges. This roll may also be positioned to effect a bending of the strand as it emerges from the bath, pre senting a flatter strand to the gaseous jet nozzles and more accurately centering the strand therebetween, thereby improving coating weight distribution.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic illustration of a portion of a hot dip metallic coating process showing one embodiment of this invention.

FIG. 2 is a schematic illustration similar to FIG. 1 but showing a modification of the invention.

FIG. 3 is a schematic, top plan view of a coating bath showing one aspect of the invention.

FIG. 4 is a schematic illustration showing a modification of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT This invention is directed to a particular, novel practice and apparatus utilized in conjunction with a conventional hot dip metallic coating operation. It is to be understood and emphasized at the outset that the conventional aspects of such a metallic coating operation do not per se form a part of this invention; however, it will also be understood that the teachings of such conventional practices must be followed in conjunction with this invention, in order to achieve the desirable results set forth herein.

According to known techniques, the first step in any hot dip metallic coating operation involves the thorough cleaning of the surface of the strand to render it receptive to the molten coating metal. By way of example, the Sendzimir process, first taught in US. Pat. 2,110,893, and now in widespread use, contemplates successive heat treatment of the strand in an oxidizing and a reducing atmosphere. According to this process, the heat treatment in the oxidizing atmosphere is effective to form a thin, homogenous oxide coating on the strip as it passes through the furnace. When subsequently treated in a reducing atmosphere, any previously formed oxide on the surface is reduced to a thin, pure coating of nascent iron, which is of course readily receptive to all of the commonly used metallic coating metals.

It should be understood that the foregoing method of preparation is exemplary only. Other cleaning methods such as the pickle and flux process, or various other wet cleaning actions may be utilized.

In any event, the prior art metallic coating practice contemplates that the cleaned strand is maintained in a protective atmosphere, and passed into a bath of molten coating metal. Exemplary coating metals include zinc, aluminum, terne, or alloys thereof.

Finally, according to conventional practice, the strand is withdrawn from the bath of molten coating metal, and a finishing operation is carried out.

Referring now to FIG. 1, a bath of molten coating metal is schematically indicated at 10. The normal level of molten metal in the bath is indicated at 12, and the strand to be coated is indicated at 14.

It will be understood that prior to arriving at the portion of the process schematically indicated in FIG. 1, the surface of the strand 14 will have been thoroughly cleaned. At this point, the strand 14 enters the snout 16 which will be provided with a suitable protective atmosphere. The strand 14 is then passed downwardly into the bath and around the pot roll 18. After emerging from the bath, it will be seen that the strand passes vertically upward in a substantially straight path of travel, between the opposedgaseous jet finishing nozzles 20 and 22. One form of finishing nozzle suitable for the practice of this invention is shown in copending application Ser. No. 841,936 entitled Metallic Coating Method.

The instant invention contemplates the provision of a roll 24 which is only partly submerged in the bath, and may be positioned with respect to the pot roll 18 and the path of travel of the strand so as to effect a bending as the strand emerges from the bath. The penetration of roll 24 is about /2 to 6 beyond the vertical passline.

It is important that the partly submerged roll be rotated in the direction of strip travel. It may be mounted for free rotation, so that it is driven in rotation by frictional contact with the stri 14, or it may be externally driven.

The diameter of the roll is not particularly critical. The invention has been successfully practiced on a commercial coating line utilizing a neight inch diameter stainless steel roll.

The extent to which the roll 24 is submerged in the molten metal bath may vary within certain limits. The centerline of the roll 24 must be below the bath surface. The most important factor is the distance between the surface of the strand 14 and the surface of the roll 24 when measured at the molten metal level 12. Obviously, this parameter will take into account both roll diameter and the extent of submersion. Empirical investigations established that this distance may be varied between oneeighth inch and about three inches.

FIG. 3 shows schematically one of the results obtained by virtue of the partlysubmerged roll 24. In this figure, the coating pot is again schematically indicated at 10, the strip being coated at 14, and the partly submerged roll at 24. The dashed lines 24a indicate the full diameter of the roll 24, and it will be observed that the lower line 24a is substantially in contact with the strip 14. The solid lines 24b indicate the lines of contact between the molten metal in the bath and the surface of the roll 24. The dimension indicated at 26 represents the distance between the surface of the strand 14 and the surface of the roll 24 when measured at the normal level of molten metal, all as described earlier. 7

As indicated-earlier, the surface of the molten metal in the bath 19-is normally covered with a heavy, oxide scum. When using the partly submerged roll 24 according to this invention, this oxide scum extends from the periphery of the tank into the solid line 28. The surface of the bath inside the line 28 is bright and oxide free.

By comparison, the curved, dotted line 30 below the strip 14 indicates the extent of the heavy oxide cover (and the outer limits of the bright oxide free bath area) when the roll 24 is not used. Of course, in such a case, the limits of the bright bath area would be identical on both sides of the strand 14.

By comparing the dotted line 30 with the line 28, it will be seen that the roll 24 provides a bright Oxide free bath area extending on both sides of the strand 14, and well beyond the edges. Thus, when the strand is withdrawn from the bath upwardly, there is no tendency to pick up oxide scum or the like, and berries on strip edges are eliminated.

It will be apparent from the foregoing that the partly submerged roll 24 does not, in contradistinction to conventional exit rolls, limit the quantity of molten metal applied to the strand. While not wishing to be bound by theory, it is believed that the rotating action of the roll, partly submerged in the molten metal bath, serves to effect a pumping action which forces surface oxides away from the exit area of the bath.

The rotating roll creates a turbulence which, combined with the gas flow from the jet nozzle on the roll side of the strip, develops a surface flow on the bath at the strip edge. This flow appears to emanate radially from the oint where the strip exits the bath and produces .the dogbone shaped clear bath area defined by 28.

The schematic showing of FIG. 4 ilustrates another embodiment of the invention, wherein the roll 24 does not deflect the strip or in fact contact the strip 14. It will of course be apparent that the roll 24, in this embodiment, must be externally driven in rotation, so as to provide the bright oxide free exit area at the surface of the bath. As with all embodiments of the invention, the,finishing is carried out by the gaseous jet nozzles 20 and 22.

While not specifically shown, it is clearly within the scope of this invention to provide a pair of partly submerged rolls, one arranged on each side of the strand as it emerges from the bath, and submerged to the extent set forth earlier in this specification.

FIG. 2 shows another embodiment of the invention. It will be apparent that the reference numerals 10 through 24 in this figure correspond exactly with the reference numerals in FIG. 1. It will be observed that this embodiment differs from that shown in FIG. '1 in the provision of an additional roll 32 fully submerged within the bath, and positioned with respect to the rolls 1S and 24 to define what might be considered a three roll bender. It may be adjusted to penetrate the passline from 0 to 6 inches;

In this embodiment, it will again be noted that the roll 24 is partly submerged in the coating metal bath, and positioned so as to effect a bending of the strand 14 a it is withdrawn from the coating metal bath.

It will be apparent by examining FIGS. 1 and 2 that both embodiments illustrated are designed so as to effect a reverse bending of the strand 14 at least once before the strand passes between the gaseous finishing nozzles 20 and 22. This reverse bending at the coating bath temperatures has been found to be effective in presenting an extremely flat Strip surface to the jet nozzles 20 and 22, thereby greatly improving the uniformity of the coating across the strip width. In addition, the (flatter presentation of the strip permits for accurate control of coating Weight distributions across the strip width as by contouring the jet nozzle openings as taught in the copending application referred to earlier.

It is believed that the foregoing constitutes a full and complete disclosure of this invention. No limitations are intended or implied except insofar as set forth in the claims which follow:

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a process for coating a ferrous base strand with a molten coating metal including at least the steps of thoroughly cleaning the surface of said strand torender it receptive to said molten coating metal, passing said cleaned strand into a bath of molten coating metal, withdrawing said strand from said bath, and thereafter solidifying the molten coating metal adhering to said strand the improved process characterized by the steps of:

(a) providing a roll only partly submerged in said bath to the extent that the centerline of said roll is below the surface of said bath and the distance between the surface of said strand and the surface of said roll as measured at the normal level of molten metal in said bath is at least /8 inch;

, (b) withdrawing said strand from said bath past said roll;

(c) rotating said roll in the direction of strand travel whereby to provide a bright, oxide free bath surface area through which said strand emerges from said bath;

((1) finishing the molten coating metal adhering to said strand with an elongate gaseous jet.

2. The process claimed in claim 1, wherein said rotating roll is positioned so as to effect a bending of said strand as it is Withdrawn from said bath past said roll.

3. The method claimed in claim 1 including the provision of at least a second roll fully submerged in said bath, said second roll being positioned with respect to said first roll and the path of travel of said strand whereby to effect the reverse bending of said strand.

4. The method claimed in claim 3 including the provision of a third roll fully submerged in said bath, said third roll being positioned with respect to said first and said second rolls and the path of travel of said strand whereby 6 to bend said strand in one direction, then in the opposite direction, and then in the original direction.

References Cited UNITED STATES PATENTS 2,224,578 12/1940 Wean et a1 117-114 AX 3,499,418 3/1970 Mayhew 117-102 MX 3,287,158 11/1966 Whitfield 117-115 X 3,066,041 11/1962 Busch 117-102 MX 2,950,215 8/1960 Slater et al 117-102 MX 2,435,058 3/1944 Matteson 117-114 A 2,686,135 8/1954 Butler 117-102 M 2,702,525 2/1955 Whitfield 117-114 CX 2,992,941 7/1961 Whitley et al 117-102 M FOREIGN PATENTS 956,954 4/1964 Great Britain 117-114 C ALFRED L. LEAVITT, Primary Examiner J. R. BATTEN, JR., Assistant Examiner US. Cl. X.R.

117-114 A, 114 B, 114 C, 114 R; 118-63, 420 

